Among the requirements for proper redirection and shaping of light within an optical apparatus is the requirement that lenses, mirrors, and other optical components have a desired spatial orientation with respect to each other. Many optical assemblies require precision mounting techniques to assure alignment between components with up to five degrees of freedom. An example is a Cassegrain telescope where the primary mirror and a secondary mirror need to be precisely aligned to each other. The same problem is faced with similar catoptric or all-reflective telescope designs and with catadioptric optical systems that employ a mix of reflective and refractive components, as well as more generally with objective lenses and components designed using similar optical principles.
A number of optical apparatus take advantage of the reduced image aberration and other benefits that are available with substantially monocentric or concentric designs. This principle is used in optical apparatus designed using Cassegrain, Officer, and Schwartzchild models, familiar to those skilled in the optical design arts.
By way of example, FIG. 1 shows an arrangement of optical components modeled after a Schwartzchild design and used to form a microscope objective 10. A secondary mirror 14 is spaced apart from a primary mirror 12. Both the concave primary and the convex secondary mirrors, 12 and 14 respectively, have substantially the same center of curvature C. An aperture hole A is formed in the center of primary mirror 12, providing a path for collimated light to secondary mirror 14.
One approach to mounting secondary mirror 14 in precise relationship to primary mirror 12 is to employ a “spider” support, the term commonly used for a mechanical mount with an arrangement of legs or struts that project radially outward from the secondary mirror to a supporting structure. For optics mounted within a cylindrical housing, the legs or struts of the spider support are typically fastened along the inner walls of the housing. Alternately, the spider support can suspend the secondary mirror from a support structure that is provided for the primary mirror or from points on the inner periphery of the primary mirror, as shown, for example, in U.S. Pat. No. 7,274,507 entitled “Two-Mirror Telescope with Central Spider Support for the Secondary Mirror” to Stenton et al.
A known technique for device fabrication is to machine the primary mirror surface, a flange orthogonal to the optical axis, and a pilot diameter in one operation. The spider support is then similarly machined to mount on the flange and fit the pilot diameter to a tolerance to assure proper alignment to maintain optical performance. These tolerances typically require special measurement equipment, and the machined components can be subject to unwanted binding or “galling” at assembly.
Precision alignment of the optical surfaces for the catoptric apparatus of FIG. 1 requires adjustment for centering and tilt. Decentration, in which the secondary mirror 14 is shifted off-axis but within its x-y plane, is unacceptable for many applications and correct alignment for centering is needed. Centering requirements for device centering can have resolutions of less than a few microns. Tilt about the center C is less critical for such systems, although it may cause vignetting. Tilt about the vertex of secondary mirror 14 can be a more significant problem. For many optical systems using this type of substantially concentric optical arrangement, centering and tilt alignment can require the work of a trained optics technician, working with an interferometer or other suitable instrumentation. This adds cost and complexity to manufacture. In addition, the use of fasteners and adjustment mechanisms for device alignment can introduce other problems, such as parasitic motion. Where extremes of temperature are encountered in device application, differences in coefficients of thermal expansion (CFE) can also jeopardize alignment.
There is, then, a need for an optical element mount for concentric optical components that simplifies the complexity and expense of component alignment.